The present invention relates to catalysts for the polymerization of ethylene and, more particularly, to Ziegler type catalysts comprising an aromatic silicon compound.
A large number of different Ziegler type catalysts are already known for the polymerization of ethylene and .alpha.-olefins. These catalysts generally comprise the catalyst component itself consisting of at least one halogenated compound of a transition metal of Groups IV to VI of the Periodic System, and an activator selected from hydrides and organometallic compounds of the metals of Groups I to III of the Periodic System. The catalyst component comprising the transition metal may be fixed, if desired, on an inert support such as alumina, silica, magnesium oxide, halides of magnesium or manganese, etc.
French Patent Specification No. 2 392 044 describes the polymerization of ethylene, at a temperature of between 0.degree. and 150.degree. C. and under a pressure of from 1 to 50 atmospheres, with a catalyst component obtained by treating VOCl.sub.3 successively with triphenyl silanol, so as to form a compound having the formula (.phi..sub.3 SiO).sub.n VOCl .sub.3 (n being 1, 2 or 3), and then with an aluminum alcoholate. Japanese Patent Application No. 79/70 385 describes the polymerization of propylene with a catalyst component obtained by treating titanium tetrachloride with butyl-magnesium chloride in solution in the presence of an electron donor, such as methyl benzoate, and in the presence of 0.033 molecule of triphenyl silanol per atom of titanium.
On the other hand, it is known to polymerize ethylene under a pressure of between approximately 400 and 2500 bars and at a temperature of between approximately 180.degree. C. and 300.degree. C. Within the scope of such a process, the aim is to improve, on the one hand, the catalytic yield and, on the other hand, the quality of the polymer produced by modifying parameters, in particular: density, molecular weight, molecular weight distribution and formation of the dimer 1-butene.